It is well known in the art to separate air into nitrogen and oxygen-rich products and also potentially an argon-rich product by cryogenic distillation. In accordance with such method, the air is compressed and purified and then cooled to a temperature suitable for its distillation within a heat exchanger against return streams that comprise the nitrogen and oxygen-rich products.
The separation of the air into the oxygen and nitrogen-rich products takes place within an air separation unit having higher and lower pressure columns that are operatively associated with one another in a heat transfer relationship, typically by a condenser-reboiler located at the bottom of the lower pressure column. The incoming air is rectified within the higher pressure column to produce a crude liquid oxygen column bottoms and a nitrogen column overhead that is condensed by the condenser-reboiler to reflux the higher pressure column. A stream of the nitrogen-rich liquid is also introduced into the top of the low pressure column to reflux the lower pressure column. A stream of the crude liquid oxygen is also introduced into the lower pressure column for further refinement and to produce an oxygen-rich liquid column bottoms in the lower pressure column that is vaporized by the condenser-reboiler. A waste nitrogen stream is withdrawn below the top of the lower pressure column together with a nitrogen-rich vapor column overhead that are introduced into a heat exchanger to cool the incoming air.
It is known to produce a high pressure oxygen product by pumping a liquid oxygen stream that is composed by the oxygen-rich liquid column bottoms and then vaporizing it in a heat exchanger against a stream of the compressed and purified air that has been further compressed by a booster compressor. The boosted pressure stream of air either liquefies or is converted into a dense phase fluid against vaporizing the pressurized liquid oxygen stream to produce the high pressure oxygen product. Additionally, it is also known that a nitrogen product composed of the nitrogen-rich liquid produced in the higher pressure column can also be pumped and vaporized in a like manner.
As mentioned above, an argon product can also be separated by withdrawing an argon-rich vapor stream from the lower pressure column and rectifying it in an argon column. The resulting liquid column bottoms is returned to the lower pressure column. The argon column is refluxed by condensing argon-rich column overhead in a condenser through indirect heat exchange with all or part of the crude-liquid oxygen stream before its introduction into the lower pressure column.
Although the above process and apparatus can utilize a single, main heat exchanger for cooling the incoming air streams through indirect heat exchange with the return streams that contain the oxygen-rich and nitrogen-rich products as well as the pressurized, pumped oxygen stream, it is also known to separately vaporize the pressurized oxygen product within a separate high pressure heat exchanger. Such process and apparatus are shown in Linde Reports on Science and Technology, “The Production of High-Pressure Oxygen”, Springmann (1980). In this paper it is also illustrated to utilize the waste nitrogen stream after having been used in subcooling duty as a feed to both the higher pressure heat exchanger that is used in vaporizing the pressurized and pumped liquid oxygen and also as a feed to the other heat exchanger that operates at a lower pressure to cool the main air stream to a temperature suitable for its rectification. This waste nitrogen feed to the heat exchangers is necessary for thermal balancing purposes. By “thermal balancing” what is meant is that the waste nitrogen streams decrease the difference between warm end temperatures of the streams exiting the lower pressure heat exchanger and the higher pressure heat exchanger to inhibit warm end losses of refrigeration by such heat exchangers and also to decrease the temperature difference of the boosted-pressure air stream and the main air stream at the cold end of the high pressure heat exchanger and the low pressure heat exchanger. In this way, the temperature difference between the boosted-pressure air stream and the pumped liquid oxygen stream at the cold end of the higher pressure heat exchanger can be optimized. It is advantageous to decrease the temperature difference at the cold end of the higher pressure heat exchanger in that the boosted pressure air liquefies within such heat exchanger and then thereafter, must be expanded for its introduction into at least the lower pressure column but also, potentially, the higher pressure column. If the temperature of this stream is too warm, vapor will evolve from the boosted pressure air during the expansion to effect the requisite distillation of the air to produce the desired products.
Brazed aluminum heat exchangers are used from both the higher and lower pressure heat exchangers. Such heat exchangers have a layered construction in which each of the streams, for example the incoming air stream, the nitrogen-rich stream and etc. pass through separate layers that are arranged in a pattern to efficiently conduct indirect heat exchange between the streams. The layered construction is produced in such heat exchangers by a series of parallel parting plates and peripheral side bars to seal the layers along their edges. Manifolds are provided to feed the streams into the layers. An arrangement of fins is provided in each of the layers that increase the area available for the heat exchange.
As can be appreciated, a high pressure heat exchanger for pumped liquid oxygen service in which typically the oxygen is to be supplied at 450 psia require air at a pressure of 1100 psia to vaporize the oxygen. Heat exchangers designed to handle such high pressures are more expensive than heat exchangers designed for lower pressure duty. For example, in case of brazed aluminum plate-fin heat exchangers, such heat exchangers require the use of reduced cross-sectional areas, have a very limited selection of heat transfer fins and require thicker design elements such as parting sheets and side bars as compared with a heat exchanger that operates at a lower pressure. All of this increases the cost of such heat exchangers that are designed to operate at high operational pressures such as is the case where a pressurized, pumped liquid oxygen stream is to be vaporized. Thicker materials and other known considerations would increase the costs of other types of heat exchangers such as like spiral wound, printed circuit and stainless steel plate-fin heat exchangers.
A spiral-wound heat exchanger is in general a tubular heat exchanger, wherein copper or aluminum tubes are wound round a central mandrel. The tubes and mandrel are enclosed in a pressure vessel shell. Each tube starts and ends in one of several tubesheets which are connected through the pressure vessel shell to headers. There will be one inlet and one outlet header for each stream in the heat exchanger.
If the operating pressure is high, these exchangers must utilize thicker tube walls to contain the pressure, which increases the quantity of material required. Hence spiral wound heat exchangers are more expensive if required to operate at higher pressure. Diffusion-bonded heat exchangers are constructed from flat metal plates into which fluid flow channels are either chemically etched or pressed.
Plates are then stacked and diffusion-bonded together by pressing metal surfaces together at temperatures below the melting point, to form a block. The blocks are then welded together to form the complete heat exchange core. Headers and nozzles are welded to the core in order to direct the fluids to the appropriate sets of passages. Design pressures up to 600 bara can be accommodated.
Higher design pressures are achieved in a printed circuit heat exchanger at the expense of smaller channels with thicker walls. To achieve the same pressure drop and heat transfer duty more material will be required—hence the heat exchanger is more expensive.
As will be discussed among other advantages of the present invention, a method and apparatus is provided for separating air in which fabrication costs of the higher pressure heat exchanger can be reduced by decreasing its size.